We are using the isolated rabbit testis perfused with artificial media to examine the control of androgen secretion. We have recently shown that testosterone tritium is metabolized intratesticularly to dihydrotestosterone, 5 alpha androstan-3 alpha, 17 Beta-diol and 5 alpha androstan-3Beta, 17 Beta-diol which are subsequently secreted into the testicular venous effluent. Surprisingly, testosterone only accounted for 65% of the total mass of these four androgens secreted by the testis in situ or in vitro perfused rabbit testis. Experiments are presently underway in which we are testing the effect of germinal epithelium destruction by heat and the effect of testicular maturation upon the secretion of testosterone, dihydrotestosterone, 5 alpha androstan-3 Beta, 17 Beta-diol and 5 alpha androstan-3 Beta, 17 Beta-diol by perfused rabbit testes.